Red Giants and Planet Formation

This article will explore the potential for life to develop in the outer planetary systems of red giant stars. It will then discuss the death-throes of red giant stars, and whether the subsequent outward thrust of stellar material might provide another mechanism for free-floating planets in interstellar space.

Exoplanets have already been found orbiting extremely old stars, one some 11 billion years old (1). This star, named Kepler-444, makes our own Sun, at a mere 4.6 billion years old, seem like an infant in comparison. The implication of this is that life could readily have got going early on in the history of the universe, long before the birth of our Sun. Furthermore, if these exoplanets were to benefit from a relatively stable stellar environment during that long timescale, then the chances of life evolving into higher forms are statistically more probable. Scale this up across trillions of stars, and the possibilities become clear.

Our own Sun has a shorter lifespan than this. Its main sequence life is expected to last another 5 billion years, by which point it will have burned up all of its hydrogen fuel. Then it will swell into a red giant star, before collapsing down into a white dwarf. For Earth, this post-main sequence (post-MS) phase of the Sun’s life will be pretty disastrous. The Sun’s expansion to a red giant will swallow the Earth up. However, a less catastrophic outcome might be expected for planets in the outer solar system, beyond, say, Jupiter. In fact, their climates might significantly improve – for a while, at least. The habitable zone of the solar system will expand outwards, along with the expanding star. Saturn’s largest moon Titan, for instance, might benefit greatly from a far milder climate – as long as it can hang onto its balmy atmosphere in the red heat of the dying Sun.

The expansion of habitable zones, as late main sequence stars become hydrogen-starved, offers the potential for life to make a new start in previously frigid environments. The burning question here is how long these outer planets have to get life going before the red giant then withdraws into its cold white shell. A study published last year by scientists at the Cornell University’s Carl Sagan Institute attempted to answer this question (2), choosing to examine yellow dwarf stars whose sizes range from half that of the Sun, to approximately twice its mass. They argue that the larger stars along this sequence could well have larger rocky terrestrial planets in their outer planetary systems than our Sun does (at least, insofar as we know it does!) This is because the density of materials in their initial proto-planetary disks should be that much greater for larger stars (3). Larger Earth-like planets in outer regions mean more potential for stable atmospheric conditions during the post-MS period under consideration. In other words, the growing red giant (which is shedding its mass pretty wildly at this point) would not necessarily blast away an outer planet’s atmosphere if that rocky planet had sufficient gravity to hold onto it.

However, the larger stars enjoy much shorter post-MS phases than their cooler cousins. So the potential for larger Earth-like planets in the outer reaches of their surviving planetary systems is offset by the shorter time period required to allow life to get a foothold. That might not be as much of a problem as it first appears, however:

“Life may become remotely detectable during the post main sequence lifetime of a star. First, life may be able to evolve quickly (i.e. within a few million to a hundred million years). Secondly, it is not necessary for life to evolve during the post-MS phase. Life may have started in an initially habitable environment and then moved subsurface, or stayed dormant until surface conditions allowed for it to move to the planet’s surface again, like in a star’s post-MS phase. Lastly, life could have evolved during early times on a cold planet located beyond the traditional habitable zone, remaining subsurface or under a layer of ice until emerging during the post-MS phase.” (3)

That said, the cooler main sequence stars (those with a fraction of the Sun’s mass) enjoy a much, much longer post-MS period – a ‘retirement’ period which might last as long as 9 billion years! But, these same stars also enjoy very long main sequence lifetimes, meaning that some of the oldest, coolest stars have yet to reach the point where they might have burned up all of their hydrogen fuels within the actual lifetime of the universe:

“None of the cool late K [orange dwarf] and M [red dwarf] stars have yet reached the post-MS phase, making the lifetime in the post-MS HZ for cool stars a prediction, not an observable quantity.” (3)

So, we need not concern ourselves particularly with orange dwarfs and red dwarfs – the Sun’s smaller stellar cousins. They simply take too long to burn out. The bigger the star, the quicker it moves through its lifecycle (to complicate matters further, this is also dependent upon its inherent metallicity, which tends to be greater in stars as the universe ages). So, stars larger than the Sun move through their post-MS period much quicker than their cooler cousins (4). That period of time between the fully expanded red giant phase, and the essentially deceased white dwarf phase, contains mysteries which have yet to be unravelled. Whilst discussing the decreasing size of the famous red giant star Betelgeuse, Edward Wishnow, a research physicist at UC Berkeley’s Space Sciences Laboratory, was quoted making the following point about end stage red giants:

“Considering all that we know about galaxies and the distant universe, there are still lots of things we don’t know about stars, including what happens as red giants near the ends of their lives.” (5)

The red supergiant Betelgeuse is currently shedding huge amounts of itself into space, forming a spectacular planetary nebula (6). These nebulae are created around red giants as they move relatively rapidly through a sequence of internal changes. They are essentially throbbing and pulsating back and forth as the increasingly unstable red giant star expands and contracts in upon itself, as it feeds upon an increasingly heavier diet of internal nuclear fuels:

“During the latter parts of the Red Giant stage of a star, the star begins to throb and pulsate. The helium-burning shell collapses into the core when its contents are fused into carbon. There is a brief shut-down of one form of nuclear fusion and the star shrinks slightly. Then a new shell of helium ignites and blows the star outward. This shrinking and expanding is called the Asymptotic Giant Branch lifestage of a star, and during this time, the star sheds much of its outer material into space in huge rings of gas and dust.” (7)

And this is where things get interesting. The image above, of the red star surrounded by a encircling planetary nebula, is of the red giant V838 Monocerotis, in the constellation Monoceros. This red giant is blowing its outer layers into space without actually turning into a nova (8). It appears like a red star wrapped up in a dusty nebula. Similar, then, to the imagery I have described for a Dark Star in our own backyard. However, this is on a titanic scale, at the dying end point of a star’s life. But it creates an interesting precedent for the kind of structure I’ve been discussing for a much, much smaller red object, closer to home; itself perhaps wrapped up in a cloud of obscuring dust (9, 10, 11). Let’s explore this connection further.

Non-conventional Planet Formation

Could massive ‘clumps’ of the red giants’ planetary nebulae get interred into interstellar space in great numbers, and after drifting through the darkness of interstellar space, end up getting picked up by the gravitational fields of main sequence stars, like the Sun? I wonder whether the materials driven forth by the red giant could be a source of clumps of dark interstellar material sizeable enough to form massive gaseous planets within them, like Dark Stars and other sub-brown dwarfs.

This stunning picture of the planetary nebula in Monoceros relied upon a chance flaring of red giant starlight illuminating the dusty nebula beyond, a phenomenon known as a ‘light echo’ (8). It may only have been blind luck that this image was even captured. As the red giant dies back, and in the absence of other illuminating sources, this nebula will become progressively darker. Arguably, then, dark nebulae may be emerging from these dying stars regularly, but never witnessed by astronomers.

Back in January, I wrote about the ‘spaghettification’ of stars by black holes, and how this debris field of matter is flung out into the rest of the Milky Way by the supermassive black hole which lies at the galactic centre (12). Clumps of this strewn material, or ‘spitballs’, are thought to become sizeable free-floating planets (13), including sub-brown dwarfs. Coming back to end-stage red giants, might not these expanding planetary nebulae also provide a non-conventional environment for planetary formation, as matter clumps together in the eddies of this outwardly expanding rush of material? Chips off the old block, one might say.

The more we come to appreciate how much material is ejected from young planet-forming star systems, binary star systems (14), star-crushing black holes and red giants, the more we need to accept that interstellar space is far from empty. On the contrary, like a swirling junk yard, it is a vast repository of broken stars, planets and dark nebulae, consisting of material which is either the unfinished detritus of creation, or from repetitive cosmic recycling. This non-uniform stream of material moves around the galaxy, like the stars – but is not sufficiently lit by them to be observable.

The stars clear away this material from their own local environments through the action of their solar winds, their heliopause borders and the gravitational wells of the stars themselves. As we happen to exist within one of these solar bubbles, and our subjective viewpoint is taken from within them, we assume a great deal about the conditions beyond (11). Because we cannot see stuff out there, besides the obvious denser patches lit internally by stars (like star-forming nebulae, and giant molecular clouds), we conclude that interstellar space is largely empty. But then, where does all this junk end up? I suspect much of it aggregates into sub-stellar free-floating planets, forming dark mini-systems enveloped in shrouds of gas and residual matter. So, instead of an opaque fog of matter strewn across space, which we might readily observe, there are instead tiny rain drops of condensed matter invisible in the starlight. A galaxy full of dark stars and free-floating planets; felt, but unseen.